Planck's law is a mathematical relationship for the spectral radiance emitted by a blackbody (i.e. a body that absorbs all radiant energy falling on it) at a given temperature as a function of frequency or wavelength. From another point of view it can be used to define a black-body as a body emitting radiation following Planck's law. The model of black-body is fundamental to simplify the description of the radiation thermally emitted by a generic body at a pre-fixed temperature and wavelength as the product of its (specific) spectral emissivity and the value predicted (at the same wavelength) by the Planck's law for a black-body at the same temperature. This way the radiation thermally emitted by a generic body can be expressed just as a (specific, as modulated by the spectra emissivity) fraction of the one expected for a black-body. Wien’s displacement law is the relationship between the temperature of a blackbody and the wavelength at which it emits the most radiation. Wien found that the product of the peak wavelength and the temperature is an absolute constant. As far as the temperature T of the blackbody increase the intensity of the emitted e.m. radiation increases being, at whatever wavelength, grater than the one emitted by a blackbody at lower temperature (Planck). As far as the blackbody temperature increases its maximum emission occurs at lower and lower wavelengths. Wien's law is fundamental both in the selection of the spectral bands more appropriate for observing specific phenomena as well as for remotely retrieve temperature of far objects by the analysis of the emitted spectral radiances.